Our Sun. III. Present And Future; The Astrophysical Journal 418:457-468, 1993 Nov 20 Authors : I.-Juliana Sackmann; Arnold I. Boothroyd; Kathleen E. Kraemer

Ultimately, how long can life exist on Earth? What will happen to the Sun?

Our Sun. III. Present and Future.

The authors compute a dynamic evolving model of the Sun using modern Los Alamos interior opacities and Sharp molecular opacities beginning pre-main-sequence with gravitational contraction to the Hayashi track. The authors fit the model to measured Mass (M =1.9891x 10^33 g), Luminosity (L (sol) = 3.854x10^33 ergs/s), Radius (R = 695980 km), and age (4.55 billion years), with gross physical elemental composition of the Sun at present. This fit uses a beginning value of 27.4% helium by mass, and 1.95% by mass of elements heavier than hydrogen and helium.

The authors then compute our Sun's future, seeing significant events ahead for the Sun consistent with events observed in stars whose age and physical attributes match those of the Sun's computational model. The model has a central core temperature of 15,430,000 degrees Kelvin, a central core density of 145.7 grams per cubic centimeter, and a core pressure of 2.269 x 10^17 ergs per cubic centimeter. Most of the Sun’s active future is a long gradual transformation of hydrogen to helium on the main sequence.

The Sun's luminosity increases from its turn-on ignition value of 70% L(sol) 4.5 billion years ago to 220% L(sol) nearly 6.5 billion years from now. This main sequence phase lasts nearly 11 billion years. However, from now, in 1100 million years, the Sun's luminosity will be 110% of the present value. In 3500 million years, the Sun's luminosity will become 140% of the current value.

At 110% L(sol), Earth and Earth’s life forms will experience a ‘hot moist greenhouse’ which will continue to become hotter and moister. From approximately this time forward, Earth’s water enters the final stages of its existence on the Earth’s surface. Earth’s water will gradually be evaporated, and most of its water-bound-hydrogen escape to space as the molecules are dissociated high in the Earth’s atmosphere by the energy of the brightening Sun. When the Sun warms to the 140% L(sol) the oceans will evaporate completely in an ‘irreversible runaway greenhouse.’ [Water vapor's role as the major greenhouse gas will yield to carbon dioxide as the amount of water vapor (gas) in the high atmosphere is depleted. -- Les Porter]

Most Earth life (as we know it) will end between 1.1 billion and 2 billion years from now. Ultimately, life will exist for absolutely no more than 3.5 billion years free on Earth's surface from the present. That is roughly the time life has already existed on Earth. (Earth life's lifespan then is roughly a total of 7 billion years.)

In another 3.5 Billion years, all the free water of Earth will have evaporated. The water you have borrowed for your life will have been turned to mist, to gas and dissociated, hydrogen to space, oxygen to a carbon bind or a sulfur bind or some other bind. (Reviewers note: If we explore Venus, we may find traces of life before the planet's runaway greenhouse burned it all and lost the water, hydrogen to space, oxygen to the carbon dioxide atmosphere that is 90 times as heavy as Earth's atmosphere and devoid of water. Call it 900 F, and oppressive. Venus' atmospheric mass is now a little less than what the carbon dioxide atmospheric mass of Earth will be in the future.)(1)

The greenhouse disposition of the Earth's free nitrogen would change only it's atmospheric partial pressure and what is now 78% by volume would become less than 4% of the total in the post-water era. Since O2 is so reactive at the temperatures the gases of earth will experience with the warming Sun, there will be very little to no measurable free Oxygen. The Argon present in Earth's atmosphere will still be here mixed with the heavy inert CO2 but would be a very minor component by mass of the Earth's entire atmosphere.

Being on Venus' surface now is like experiencing the pressure of Earth's ocean, about 3000 feet down, beneath the surface. Dive to the surface of Venus, anyone, in your hot/cold, wet/dry suit? Getting rid of the heat to make what humans must have as livable temperature for a time of more than a few hours would require some significant engineering, and will probably not occur for a very long time if ever. Likely, we humans may mine Venus, if needed, but it will be done remotely. Eventually Earth will become like Venus, at least for a few billion years.

When the Sun reaches the end of its main sequence burning, its radius has increased to 1.575 times it’s current value. It’s core hydrogen is essentially depleted, and with a diminished "light pressure," gravitational contraction resumes and brings more upper-layer hydrogen to the core and into conditions where it begins fusing vigorously and the Sun begins to expand on the red giant branch. This happens relatively quickly and the hydrogen fuses within a larger volume and surface area around the core. A prodigious amount of matter is fused, and energy released. This happens roughly 6.5 billion years from now.

As the Sun embarks on the red giant course, its surface vastly enlarges, cools, and reddens. The Sun’s radius reaches nearly 170 times its current value, and the Sun swallows the planet Mercury within its bloated surface. The luminosity increases more than a thousand fold and near the end of its red giant branch traverse, the Sun's maximum luminosity of 2349 L(sol) powers copious ejection of mass with the Sun’s fierce solar wind carrying away more than 27.5% of its original mass. How soon and at what rate the mass ejection and solar wind carry away significant portions of the Sun's original mass determines what happens to Venus and Earth.

Throughout the Sun's red giant stage, the inner planets suffer the ravages of an intense and violent continuous solar mass ejection. The solar winds increase with the Sun's radius. By the end of the red giant stage, any remaining gaseous atmospheres on Venus, Earth, and Mars are stripped by the Sun’s shedding of mass. This loss of mass, is not negligible, and affects the Sun's ability to hold the planets. The Sun’s weakening gravitational attraction for the planets allows the planet’s orbits to migrate outward, but not enough to save the planet Mercury. Both Venus and Earth's fate rest with the mass ejections rates and timing. The amounts of mass loss in the red giant stage are nearly 30% of the Sun’s original mass.

As the Sun works toward the asymptotic giant branch (AGB) its luminosity falls temporarily from its red giant brightness, but severe mass loss continues. The AGB tenure has dual hydrogen and helium burning shells that work toward and away from each around and outward from the depleted core. Within the Sun, burning voids large shell volumes of fuel (radiation pressure diminishes in burned zones) and leads to contractions with the net result that the core temperature reaches into the 10^8 Kelvin range and core densities reach into the 10 ^ 6 grams per cubic centimeter range. When the various contractions and expansions occur, and temperatures and densities allow, helium burning ignites in shells where hydrogen had burned and been depleted just before. The interplay between hydrogen burning to helium and making helium shells and between violent explosive helium shell flashes along with helium shell expansion and quenching occurs multiple times before the Sun’s final mass losses leave the naked degenerate core of the white dwarf it finally becomes.

There is enough uncertainty in the opacity values at present, as well as which of several of the nuclear reactions that can occur as a result of opacity uncertainty at elevated temperatures, that it is currently impossible to describe precisely the Sun’s final path. For example, in the preferred model, four distinct helium shell flashes occur, pushing the Sun’s luminosity from nearly 3000 L(sol) to nearly 5200 L(sol). A fifth and final helium shell flash is possible, and both opacities and mass loss rates determine the course.

Observations of stars at similar ages and masses continues, and refinements are certain in the near future. After the huge mass losses in the red giant stage, the asymptotic giant branch events cause an additional 11% of the Sun’s mass to be shed, and much of this last major mass ejection contributes to a possible planetary nebulae with the Sun as the white dwarf degenerate core (Mass = 0.541 sol) nestled in the planetary nebula's center.

The sun as white dwarf is certain, with a surface temperature of 74,080 Kelvin, and a luminosity of 90 L(sol). Enough harsh radiation from this core will likely ionize and light a planetary nebula from within. At this stage, long before a lengthy slow degenerate radiative cooling occurs, the Sun will be 12.365 billion years old. As a white dwarf, billions, perhaps trillions of years, are yet ahead of the burned-out Sun.

Insolation W/M ^2 at Luminosity at planet. The table contains an additional value for those who do not “accept” the "turn-on" brightness of the Sun as it is computed in the best model humanity has of how the sun reached it's current state and luminosity. Please notice that I have computed Venus' solar insolation at an ignition energy output of 60% of current luminosity for our sun as opposed to the 70% ignition luminosity the Sackmann, et al. model demonstrates. Even with the "cool young sun," Venus is too hot. Think about it.

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Planetary Insolation Past and Future

Look at the hypothetical 60% value for the Earth of 820.6 watts/m2 and compare that to the Venus value of 1568.3 with a 60% luminosity "turn on value" for the Sun. Venus was intercepting more Watts/unit area than Earth will be receiving at the point Earth's oceans begin migrating into the high atmosphere and where the hydrogen unbound from water, mostly by higher enegy UV, slips into space. This occurs when the Earth starts receiving a bit over 1500 watt/meter2. Note therefore, that Venus was always receiving more than that mere 1500 Watt/unit area value since the Sun really turned on at 70% of current LSol. If it had water in its early formative years, the water would have been evaporated then dissociated to space from the instant the ignited fusion of the Sun leaped at the speed of light to reach Venus. This would happen at 60% of LSol and any turn-on luminosity higher than that. (1)

The purpose of the above table become more clear when you see it as it interfaces with my "Venus: Life Never Had a Chance" posting. This table contains the 60% luminosity value I have seen in some places in both the technical and popular press, and the reason I constructed the table from I. Sackmann, et.al., was to show that Venus never had a chance for water, let alone life. Even if the sun had started at 60% luminoisty it would have never had oceans that lasted long enough to give life a chance. It will be interesting to see what funding for Venus programs ensue. The Russian's Venera program was a remarkable feat.

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This is what the Sun 'might' look like at the end of it's span of fusion. This example is the Helix Nebula, and our vantage point lines us up looking right down the "tube" this final stage of mass ejection brought this star; that intense white dwarf blazing in the center. This is how you get a white dwarf! But the shroud of gas will dissipate, blown away into space soon enough. This shroud is a beautiful shroud, isn't it? Below I'll post one not as far along, but still beautiful in a different way.

It might be worth noting here that the lifetimes of stars are clearly long indifferent things. Stars born massive burn bright and quickly die, becoming white dwarfs or neutron stars or black holes; while red dwarf stars, much less massive than the Sun can burn slowly for hundreds of billions to trillions of years. It makes a lifetime like a human microscopic in comparison, or even a species lifespan of several millions of years, miniscule; or sharks some 270 million years old; or algae even a few billion years old -- look trivial. And unless we convince ourselves these lifespans are not as trivial as ants or algae -- and gather perspective and grasp -- our lives as mayflies will surely be trivial.

How are you going to use your allotted span?

Below is NASA's official blurb for this "planetary" nebula.

This image, taken by NASA's Hubble Space Telescope, shows the colorful "last hurrah" of a star like our Sun.

The star is ending its life by casting off its outer layers of gas, which formed a cocoon around the star's remaining core. Ultraviolet light from the dying star makes the material glow. The burned-out star, called a white dwarf, is the white dot in the center. Our Sun will eventually burn out and shroud itself with stellar debris, but not for another 5 billion years.

Our Milky Way Galaxy is littered with these stellar relics, called planetary nebulae. The objects have nothing to do with planets. Eighteenth- and nineteenth-century astronomers named them planetary nebulae because through small telescopes they resembled the disks of the distant planets Uranus and Neptune.

The planetary nebula in this image is called NGC 2440. The white dwarf at the center of NGC 2440 is one of the hottest known, with a surface temperature of nearly 400,000 degrees Fahrenheit (200,000 degrees Celsius).[I would have said 200,000 degrees K.] The nebula's chaotic structure suggests that the star shed its mass episodically. During each outburst, the star expelled material in a different direction. This can be seen in the two bow tie-shaped lobes.

The nebula also is rich in clouds of dust, some of which form long, dark streaks pointing away from the star. NGC 2440 lies about 4,000 light-years from Earth in the direction of the constellation Puppis.

The image was taken Feb. 6, 2007 with Hubble's Wide Field Planetary Camera 2. The colors correspond to material expelled by the star. Blue corresponds to helium; blue-green to oxygen; and red to nitrogen and hydrogen.

Les Porter's notes:

On another subject entirely, but the NASA Hubble note above made me decide I would remark on it.

NGC 2440 means New General Catalogue object number 2440, and if you get the list you will see it has a particular Right Ascension (RA) and Declination (DEC), Longitude and Latitude in the reference frame that helps astonomers map the sky, and relocate any object of study.

(1) I reviewed the article and then thought about the possibilities for the existence of water on Venus. I used a few of the details of this original paper to finally realize that with the Sun starting life and burning as described by the physics above -- Venus Never could have harbored water in any quantity, nor for any length of time. I do not know when man will be able to build a device to operate in the ambient temperature of Venus -- nor why we would now -- since to myself, and I hope others, Venus could never have had water long enough for life to evolve or develop. I try to explain this in another blog about Venus, here on Xomba.

Although I know it doesn't answer the obvious questions about the origin of life -- let alone the origin of life on Earth -- I am more and more subscribing intellectually to the ideas of the late Fred Hoyle and Chandra Wickramsinghe, in that I am more than beginning to think the origin of life may be quite different than has been thought.